Jucker said on Thursday that Catalonia would not be able to join the European Union on the next day after a possibly successful independence vote and would have to undergo a standard accession procedure just like the states which have joined the bloc after 2004.
"If Catalonia gains independence one day, we will respect that choice. But Catalonia will not be able to become a member of the European Union immediately after the vote [referendum]. Catalonia will be subject to the process of accession, as the states that joined us after 2004," Juncker said in an interview broadcast by the European Commission.
He stressed that Brussels would follow the decisions made by the Constitutional Court of Spain and its parliament, and not support separatism in Europe.
Catalan authorities are preparing to hold a new independence referendum on October 1 despite strong opposition from the government in Madrid.
Catalonia has been seeking independence from Spain for years. On November 9, 2014, about 80 percent of the Catalans who took part in a non-binding referendum on the region's status as part of Spain voted in favor of Catalonia becoming an independent state.
The vote was, however, ruled unconstitutional by the authorities in Madrid.